Method for signaling information by modifying modulation constellations

ABSTRACT

Methods and systems for communicating in a wireless network may distinguish different types of packet structures by modifying the phase of a modulation constellation, such as a binary phase shift keying (BPSK) constellation, in a signal field. Receiving devices may identify the type of packet structure associated with a transmission or whether the signal field is present by the phase of the modulation constellation used for mapping for the signal field. In one embodiment, the phase of the modulation constellation may be determined by examining the energy of the I and Q components after Fast Fourier Transform. Various specific embodiments and variations are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.12/319,191 filed Dec. 31, 2008 (pending) which in turn is a continuationof U.S. application Ser. No. 11/018,414 filed Dec. 20, 2004 (issued),now U.S. Pat. No. 7,474,608. Said application Ser. No. 11/018,414 claimsthe benefit of U.S. Provisional Application No. 60/536,071 filed Jan.12, 2004. Said application Ser. No. 12/319,191, said application Ser.No. 11/018,414 and said Application No. 60/536,071 are herebyincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

In today's communications industry rapid advances in communicationprotocols and techniques are common. To facilitate widespread deploymentof new systems, significant efforts are often made to ensure newcommunications techniques and systems are compatible with previoussystems and devices, referred to herein as “legacy” systems or devices.

One problem associated with designing new generation systems is that, tobe compatible with legacy systems, new generation systems often have todeal with limitations inherent in the legacy systems. For example,preamble training and signaling fields of packets for legacy wirelesslocal area networks (WLANs) are already defined. To allow coexistencebetween legacy and new generation WLANs, it is desirable to preservepreambles having legacy compatible training and signaling fields.However, since legacy preambles may not be adequately designed todescribe new generation packet structures, which may have longer lengthsand/or require different training and signaling information, it can bechallenging to quickly identify which type of packet structure, e.g.,legacy or new generation, that follows a legacy compatible preamble.

Accordingly, a need exists to be able to quickly distinguish whether apacket having a legacy compatible preamble, may have a legacy packetstructure or a newer generation packet structure. Solutions to allowingcoexistence between legacy and new generation systems are thereforedesired without significantly complicating or constraining the signalingin new generation packet structures.

BRIEF DESCRIPTION OF THE DRAWING

Aspects, features and advantages of the embodiments of the presentinvention will become apparent from the following description of theinvention in reference to the appended drawing in which like numeralsdenote like elements and in which:

FIG. 1 shows block diagrams of two example packet structures for usewith wireless networks;

FIGS. 2 a and 2 b show respective graphs of different phases for amodulation constellation in order to distinguish packet structuresaccording to one embodiment of the present invention;

FIG. 3 is a flow diagram showing an exemplary method of communicatingaccording to one embodiment of the present invention;

FIG. 4 is a flow diagram showing a method of detecting types of packetstructure of a received transmission according to an embodiment of thepresent invention; and

FIG. 5 is a functional block diagram of an example embodiment for awireless apparatus adapted to perform one or more of the methods of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

While the following detailed description may describe exampleembodiments of the present invention in relation to wireless local areanetworks (WLANs), the invention is not limited thereto and can beapplied to other types of wireless networks or air interfaces whereadvantages could be obtained. Such wireless networks include, but arenot limited to, those associated with wireless wide area networks(WWANs) such as general packet radio service (GPRS), enhanced GPRS(EGPRS), wideband code division multiple access (WCDMA), code divisionmultiple access (CDMA) and CDMA 2000 systems or other similar systems,wireless metropolitan area networks (WMANs), such as wireless broadbandaccess systems including those supported by the WorldwideInteroperability for Microwave Access (WiMAX) Forum, wireless personalarea networks (WPANs) and the like.

The following inventive embodiments may be used in a variety ofapplications including transmitters, receivers and/or transceivers of aradio system, although the present invention is not limited in thisrespect. Radio systems specifically included within the scope of thepresent invention include, but are not limited to, network interfacecards (NICs), network adaptors, mobile stations, base stations, accesspoints (APs), gateways, bridges, hubs and radiotelephones. Further, theradio systems within the scope of the inventive embodiments may includecellular radiotelephone systems, satellite systems, personalcommunication systems (PCS), two-way radio systems, two-way pagers,personal computers (PCs) and related peripherals, personal digitalassistants (PDAs), personal computing accessories and all existing andfuture arising systems which may be related in nature and to which theprinciples of the inventive embodiments could be suitably applied.

The following inventive embodiments are described in context of exampleWLANs using orthogonal frequency division multiplexing (OFDM) and/ororthogonal frequency division multiple access (OFDMA) although theinvention is not limited in this respect.

The Institute of Electrical and Electronics Engineers (IEEE) finalizedan initial standard for WLANs known at IEEE 802.11 (1997). This standardspecifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbpsusing either direct sequence or frequency hopping spread spectrum. TheIEEE 802.11 working group has since published three supplements to the802.11 standard: 802.11a (OFDM in 5.8 GHz band) (ISO/IEC 8802-11: 1999),802.11b (direct sequence in the 2.4 GHz band) (1999 and 1999Cor.-1/2001), and 802.11g (OFDM in the 2.4 GHz band) (2003). Thesesystems, most notably 802.11a and 802.11g utilizing OFDM, areindividually or collectively referred to herein as “legacy” WLANs.

The IEEE 802.11a standard specifies an OFDM physical layer that splitsan information signal across 52 separate sub-carriers to providetransmission of data. The primary purpose of the OFDM Physical Layer isto transmit MAC (medium access control) protocol data units (MPDUs) asdirected by the 802.11 MAC Layer. The OFDM Physical Layer is dividedinto two elements: the PLCP (physical layer convergence protocol) andthe PMD (physical medium dependent) sublayers. The PLCP sublayerprepares MAC protocol data units (MPDUs) for transmission and deliversincoming frames from the wireless medium to the MAC Layer. The PLCPsublayer minimizes the dependence of the MAC layer on the PMD sublayerby mapping MPDUs into a frame format (also referred to as packetstructure) suitable for transmission by the PMD.

Examples of frame formats or packet structures 100 for use in WLANs aregraphically represented in FIG. 1 and may include a preamble portion fora receiver to acquire an incoming OFDM signal and synchronize thedemodulator. The preamble may include one or more training fields and/orsignaling fields (sometimes separately referred to as headers)including, for example, a legacy short training field (L-STF), a legacylong training field (L-LTF) and a legacy signaling field (L-SIG) 114,124, which are collectively referred to herein as a legacy compatiblepreamble. The portion of packet structures 100 to follow the legacycompatible preamble may depend on whether the packet structure is alegacy packet structure 110 or a newer generation packet structure 120.

For legacy packet structures 110 one or more data fields 112 typicallyfollow the legacy compatible preamble and the rate and length (in OFDMsymbols) of the legacy packet structure 110 may be determined by areceiver from the values present in the L-SIG field 114 of the legacypreamble. However, the L-SIG field 124 may not be sufficient to describenew generation packet structures 120, such as those currentlycontemplated for adoption in the IEEE 802.11n standard for highthroughput (HT) WLAN. By way of example, reserve bits in the L-SIG fieldmay already be used by legacy devices for other purposes. Accordingly,additional signaling and/or training, generally depicted by HT-SIG block122, may be needed to define the HT packet structure and/or synchronizethe demodulator to handle the HT modulation.

However, since an L-SIG field 114, 124 may be present in all legacycompatible preambles; it may be difficult for a receiver to know whetherlegacy data 112 follows the signaling field 114 or whether additional HTsignaling or training 122 follows the signaling filed 124.

The long training symbols (L-LTF) that immediately precede the signalfield 114, 124 allow a receiver to accurately estimate the clock phaseso that demodulation of the signal field, for example, using binaryphase shift keying (BPSK), is possible.

In generating OFDM signals, encoded and/or interleaved bits may bemapped on a transmit modulation constellation, for example,constellations for BPSK, quaternary phase shift keying (QPSK), and/orvarious quadrature amplitude modulation (QAM) modulation schemes. Aninverse Fast Fourier Transform (FFT) may then be performed on the mappedcomplex values to generate an array of complex values to produce an OFDMsymbol and for which multiple symbols are joined together to produce anOFDM frame. On the receiving end, an FFT is performed to retrieve theoriginally mapped complex values which are then demapped using thecorresponding constellation and converted back to bits, decoded, etc.

Turning to FIGS. 2 a and 2 b, in accordance with one embodiment, whenthe packet has a legacy packet structure (e.g., an IEEE 802.11astructure 110; FIG. 1), a traditional modulation constellation such asBPSK constellation 210 of FIG. 2 a may be used for mapping complexvalues for one or more fields (e.g., 114, 112) of the legacy packetstructure. Further, when the packet has a newer generation structure(e.g., IEEE 802.11n structure 120; FIG. 1) the one or more fields (e.g.,124, 122) may be modulated using a modified modulation constellationsuch as a BPSK constellation 220 having a phase rotation of 90 degreesas shown in FIG. 2 b. Of course, the modified constellation 220 could beused for signaling legacy packet structures and traditionalconstellation 210 could be used for signaling new generation packetstructures if desired. In this manner, information may be signaled to areceiver without modifying preamble structures or fields of the packetsthemselves.

Constellation 220 may be referred to as a BPSK-Q or Q-BPSK constellationsince its coordinates (+1, −1) are positioned along the Q axis asopposed to traditional BPSK constellation 210 having coordinates (+1,−1) along the I axis.

The 90 degree rotation of a BPSK constellation is effective as it has nosignificant effect on the robustness of the packet field (e.g., signalfield) with the modified modulation technique. However, the phaserotation for mapping values of a modulation constellation does not haveto be 90 degrees and/or other types of modulation constellations such asthose used for QPSK modulation and the like could also be used.Consequently, the inventive embodiments are thus not limited to anyparticular modulation constellation or degree of phase rotation.

Turning to FIG. 3, a method 300 for transmitting in a wireless networkmay include modulating 325 one or more portions of a transmission usinga modulation constellation having a modified phase in order to signal areceiving device of a type of packet structure associated with thetransmission.

In certain embodiments, method 300 may include encoding bits 305 andinterleaving 310 the encoded bits. If 315 a legacy packet structure isto be transmitted, one or more of the packet fields may be modulated 320using traditional modulation constellations, such as a BPSKconstellation (210; FIG. 2). On the other hand, if 315 a new generationpacket structure is to be transmitted, one or more of the packet fieldsmay be modulated 325 using a modified modulation constellation, such asa Q-BPSK constellation (220; FIG. 2).

In certain example embodiments, there may be two types of packetstructures, a legacy packet structure substantially in conformance withan IEEE 802.11a type packet structure and a second packet structuresubstantially in conformance with an IEEE 802.11n type packet structure.In one example implementation, only the HT-SIG field (122; FIG. 1) of anHT packet structure may be modulated using Q-BPSK however, theembodiments are not limited in this manner. Further, in certainimplementations, signaling a packet type using phase rotated modulationconstellations may only be used for packets which have a data payload.

For a receiver, the decision about whether the signal field is a legacymodulation or a HT field could be made by examining the amount of energyin the I and Q components after the FFT. For example, if the Q energy isgreater than the I energy (the threshold for which may be set assuitably desired), then the receiver may determine the packet has anHT-SIG field. Otherwise it may be a legacy packet or visa versa. Sincethis decision can utilize all data modulated subcarriers, for example,at least 48 for a 20 MHz WLAN system, this affords a 17 dB processinggain resulting in a highly reliable decision. The proposed detectionscheme may only be applied to the data modulated subcarriers and pilotsubcarriers can be handled differently if desired.

Turning to FIG. 4, a method 400 of receiving in a wireless network mayinclude determining a type of packet structure associated with anincoming transmission based on an I and Q energy levels of a respectivebaseband signal.

In certain embodiments, method 400 may include performing 405 a FFT on areceived transmission and examining 410 I and Q components after theFFT. If 415 the Q energy is significantly greater than the I energy, theassociated packet field is determined 420 to be an HT-SIG field.Otherwise, it is identified 425 as being a legacy packet. The FFT valuesmay then be demapped using the corresponding modulation constellationsand converted back to bits, decoded, etc.

In an example implementation, the I and Q energy levels are used todetermine whether a phase of a binary phase shift keying (BPSK)constellation used to map the HT-SIG field has been rotated although theembodiments are not limited in this respect.

Turning to FIG. 5, an example apparatus 500 for use in a wirelessnetwork may include a host processing circuit 550 may be any componentor combination of components and/or machine readable code adapted toperform one or more of the methods described herein. In one exampleimplementation, circuit 550 may include a baseband processing circuit553 to modulate bits for at least a portion of a transmission using amodulation constellation having a modified phase in order to signal areceiving device of a type of packet structure associated with atransmission. Alternatively or in addition, baseband processing circuit553 may be configured to detect energy levels of data modulatedsubcarriers as previously described. Apparatus 500 may also include amedium access controller circuit 554 and/or a radio frequency (RF)interface 510 if desired.

Host processing circuit 550 and/or RF interface 510 may include anyhardware, software and/or firmware components necessary for physical(PHY) link layer processing and/or RF processing of respectivereceive/transmit signals for supporting the various air interfaces.

Apparatus 500 may be a wireless mobile station such as a cell phone,personal digital assistant, computer, personal entertainment device,wireless router, a network access station such as a WLAN access point(AP) or other equipment and/or wireless network adaptor therefore.Accordingly, the functions and/or specific configurations of apparatus500 could be varied as suitably desired.

The components and features of apparatus 500 may be implemented usingany combination of discrete circuitry, application specific integratedcircuits (ASICs), logic gates and/or single chip architectures. Further,the features of apparatus 500 may be implemented using microcontrollers,programmable logic arrays and/or microprocessors or any combination ofthe foregoing where suitably appropriate.

It should be appreciated that apparatus 500 shown in the block diagramof FIG. 5 is only one functionally descriptive example of many potentialimplementations. Accordingly, division, omission or inclusion of blockfunctions depicted in the accompanying figures does not infer that thehardware components, circuits, software and/or elements for implementingthese functions would necessarily be combined, divided, omitted, orincluded in embodiments of the present invention.

Embodiments of apparatus 500 may be implemented using single inputsingle output (SISO) systems. However, certain alternativeimplementations may use multiple input multiple output (MIMO)architectures having multiple antennas 518, 519.

Unless contrary to physical possibility, the inventors envision themethods described herein may be performed in any sequence and/or in anycombination; and the components of respective embodiments may becombined in any manner.

Although there have been described example embodiments of this novelinvention, many variations and modifications are possible withoutdeparting from the scope of the invention. Accordingly the inventiveembodiments are not limited by the specific disclosure above, but rathershould be limited only by the scope of the appended claims and theirlegal equivalents.

The invention claimed is:
 1. A method of decoding encoded information bya wireless communication device, comprising: receiving a data unitincluding encoded information comprising one or more variable length lowdensity parity check (LDPC) codewords from a network device; decoding alength of the encoded information from a header of the data unit;determining, based at least in part on the decoded length, the length ofeach of the one or more LDPC codewords; and decoding the one or moreLDPC codewords.
 2. The method of claim 1, wherein decoding the one ormore LDPC codewords comprises, for each of the one or more LDPCcodewords: determining a plurality of variable node values; anddetermining a plurality of check node values, corresponding to paritycheck relationships, based on the variable node values and a paritycheck matrix.
 3. The method of claim 1, wherein decoding the one or moreLDPC codewords comprises using a Bahl, Cocke, Jelinek and Raviv (BCJR)algorithm.
 4. The method of claim 1, wherein decoding the one or moreLDPC codewords comprises using a min-sum algorithm.
 5. The method ofclaim 1, wherein decoding the one or more LDPC codewords comprises usinga plurality of decoding iterations.
 6. The method of claim 5, whereinthe plurality of decoding iterations consist of eight decodingiterations.
 7. A wireless communication device, including a memory andone or more antennas, the device capable to: receive a data unitincluding encoded information comprising one or more variable length lowdensity parity check (LDPC) codewords from a network device; decode alength of the encoded information from a header of the data unit;determine, based at least in part on the decoded length, the length ofeach of the one or more LDPC codewords; and decode the one or more LDPCcodewords.
 8. The device of claim 7, wherein the device is furthercapable to decode the one or more LDPC codewords, for each of the one ormore LDPC codewords, by being configured to: determine a plurality ofvariable node values; and determine a plurality of check node values,corresponding to parity check relationships, based on the variable nodevalues and a parity check matrix.
 9. The device of claim 7, wherein thedevice is further capable to decode the one or more LDPC codewords via aBahl, Cocke, Jelinek and Raviv (BCJR) algorithm.
 10. The device of claim7, wherein the device is further capable to decode one or more LDPCcodewords via a min-sum algorithm.
 11. The device of claim 7, whereinthe device is further capable to decode the one or more LDPC codewordsvia a plurality of decoding iterations.
 12. The device of claim 11,wherein the plurality of decoding iterations consists of eight decodingiterations.